Identifying Alcohols

Source: Lara Al Hariri at the University of Massachusetts Amherst, MA, USA

1. Preparation of ReagentsExpand

   Here, we show the laboratory preparation for 10 students working in pairs, with some excess. Please adjust quantities as needed.

   • Before you begin, put on a lab coat, safety glasses, and nitrile gloves, and set up a waste container for aqueous chromium, iron, and zinc waste. Note: Work in a fume hood because the reagents are acidic, toxic, and corrosive. The reagents undergo exothermic reactions when they are mixed, so you will prepare them in ice baths.
   • Prepare 100 mL of 0.03 M aqueous iron(III) chloride. First, weigh out 0.5 g of anhydrous iron(III) chloride and pour it into a 250-mL beaker.
   • Place about 250 mL of crushed ice in a 600-mL beaker and add just enough water to cover the ice. Settle the beaker of iron(III) chloride in the ice bath and place the bath on a stir plate.
   • Measure 100 mL of deionized water and pour it into the beaker containing iron(III) chloride. Add a medium stir bar to the beaker and stir until the iron(III) chloride has completely dissolved.
   • Label a brown 100-mL glass bottle ‘0.03 M iron(III) chloride in water’. Once the iron(III) chloride has dissolved, retrieve the stir bar, and pour the solution into the labeled bottle. Cap the bottle and store it in a cool, dry, dark cabinet for acids.
   • Prepare 100 mL of Jones reagent. Measure out 75 mL of deionized water, pour it into a 250-mL beaker, and add a medium stir bar. Refresh the ice bath and chill the water in the bath.
   • Measure out 25 g of chromium trioxide in a fume hood and place it in a clean 150-mL beaker.
   • Make a second ice bath and settle the beaker of chromium trioxide in it. Put this bath on the stir plate and add a medium stir bar to the beaker.
   • Measure out 25 mL of 98% by weight sulfuric acid and start slowly adding it to the chromium trioxide. When enough sulfuric acid has been added to cover the stir bar, start slowly stirring the mixture, being careful to avoid splashing.
   • Once you have added all 25 mL of sulfuric acid, continue stirring the mixture until the chromium trioxide has dissolved completely. Then, turn off the stir motor, and remove the bath and mixture from the stir plate.
   • Now, confirm that the water in the ice bath is cold. Place that bath on the stir plate and start stirring the water at a moderate speed.
   • Use a glass Pasteur pipette to slowly add the chromium-sulfuric acid mixture to the cold water. Leave the Pasteur pipette in the empty beaker when you are done.
   • Continue stirring the mixture until the solution cools to room temperature. Then, turn off the stir motor and remove the bath from the stir plate.
   • Transfer the stir bar to the empty beaker without letting any liquid drip on the floor of the hood. Carefully pour the Jones reagent into a 100-mL brown glass bottle and cap it. Label the bottle and list its components. Then, store it in the acid cabinet.
   • Jones reagent contains hexavalent chromium, which is highly toxic and corrosive, so you must quench the residual solution before you clean the glassware. With the beaker in the ice bath, slowly add 10 – 20 mL of water.
   • Pipette water along the sides of the beakers to rinse down the residues. Then, slowly add cyclohexanol to the beaker until the solution lightens in color. Quench the residue on other glassware and tools the same way.
   • Lastly, prepare about 50 mL of Lucas reagent. Measure 47 mL of 37% by weight hydrochloric acid and pour it into a 100-mL beaker.
   • Prepare a fresh ice bath and set it on the stir plate. Place the beaker in the bath, add a stir bar, and start stirring the HCl.
   • While the HCl cools, weigh out 76.9 g of anhydrous zinc chloride. Use a spatula to slowly add the zinc chloride to the cold HCl, pausing as needed to let the mixture cool back down.
   • Continue stirring until the zinc chloride has dissolved completely and the solution has cooled to room temperature.
   • While you wait, label a 100-mL glass bottle as ‘Lucas reagent’. When the solution is ready, turn off the stir motor, retrieve the stir bar, and pour the solution into the labeled bottle. Close the bottle with an acid-resistant cap and store it in the acid cabinet.
2. Preparation of the LaboratoryExpand
   • Place bottles of 1-butanol, 2-butanol, and 2-methyl-2-propanol in a hood for solvents.
   • Put a box of Pasteur pipettes and a few pipette bulbs in the same hood and set a glass waste container within easy reach.
   • Then, pour about 20 mL of acetone in a 50-mL glass bottle. Cap the bottle, label it, and put it in the solvent hood.
   • Next, set a container of phenol by an analytical balance. Confirm that there are enough small spatulas, weighing boats, and laboratory wipes for the lab.
   • Now, randomly assign each lab group one of the four unknown alcohols: 1-propanol, 2-propanol, 2-methyl-2-butanol, and 4-chlorophenol.
   • For each group, prepare a vial of about 1 mL, or 0.5 g, of the appropriate alcohol and label it with a code name. Store the vials in the instructor's hood.
   • Set out the following glassware and equipment at each student lab station (we suggest that students work in pairs):

     13    Small test tubes

     1    Test tube rack

     3    Small vials with caps

     1    50-mL beaker

     1    250-mL beaker

     1    5-mL graduated cylinder

     1    Glass stirring rod

     1    Roll of labels or lab tape

     1    Labeling pen/marker

     Pipette bulbs

   • Fill 250-mL plastic wash bottles with deionized water and place one at each workstation. Set out a box of Pasteur pipettes in a central area for students to use at their hoods.
   • Shortly before the lab, place the three test reagents in a central fume hood. Give the unknowns to the student groups when they are ready to start the lab.
   • After the lab, collect the orange solutions containing unreduced chromium(VI) from the Jones test and quench them with water and isopropyl alcohol.